The invention relates to coating compositions that include an organic film forming resin and the cyanamide salt of a non-lead metal. The coating compositions are particularly useful for the protection of thin metallic films, such as mirrors.
A number of articles in widespread use comprise a thin metallic film or layer of silver or copper. Among the common articles on which such thin metallic films are employed are those of glass, such as mirrors. In such articles, the films are of varying thickness; for example, mirrors may have a metallic layer of 800 to 1500 Angstroms in total thickness.
Since these articles are intended for use over extended periods, the durability of the metallic film is an important consideration. These films, although varying in thickness, are always quite thin, and such thin films are subject to degradation due to oxidation and heat, and to attack by chlorides, sulfides, and other air contaminants to a greater degree than thicker films, or at least the deterioration and attack are more easily noticed at lower levels. Presumably because of the thinness of the film, even materials that are known to inhibit the corrosion and tarnishing of the metal are not sufficiently effective to prevent the degradation of such articles, and in some cases, such known "inhibitors" actually increase the rate of degradation of the film.
Because the visual properties of these articles are in most cases important, such deterioration and attack, even if slight, is highly undesirable as it is inevitably evidenced by discoloration and/or removal of the metallic film. For this reason, the exposed metallic surface of mirrors is usually coated with an organic coating composition which forms a protective layer. However, even this expedient is not always sufficient to prevent eventual attack and discoloration of the metallic film. These organic coating compositions are sufficiently permeable to various contaminants, such as chlorides, sulfides, and the like which cause and promote the deterioration to the metallic film that they do not provide adequate long term protection for the metallic films.
One common method of increasing the protection afforded to thin metallic films by such organic coating compositions is to include a leaded pigment in the compositions. Typical of such pigments are lead sulfate and lead carbonate.
An object of the present invention is to provide a composition that provides improved protection for thin metallic films and is lead-free.